A method of synchronizing wireless devices includes establishing a recurring sequence of frequency channels at which the wireless devices are to communicate. The frequency channels are divided into a plurality of groups. Synchronization information is transmitted at a respective first frequency channel in each of the groups of frequency channels during a first sampling time period. One of the groups of frequency channels is selected. A wireless device is used to sample each of the frequency channels in the selected group during the first sampling time period. non-synchronization information is transmitted after the first sampling time period. Synchronization information is transmitted at a next respective frequency channel in each of the groups of frequency channels during a next sampling time period. The next sampling time period occurs after the transmitting of the non-synchronization information.
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1. A method of synchronizing wireless devices, the method comprising the steps of:
informing each of the wireless devices of a frequency channel hop sequence at which non-synchronization information is to be transmitted;
transmitting synchronization information sequentially to each of a plurality of frequency channels in a first subset of the frequency channels included in the hop sequence;
using one of the wireless devices to sample each of the frequency channels in a second subset of the frequency channels, the second subset including one frequency channel included in the first subset and at least one frequency channel omitted from the first subset;
transmitting non-synchronization information after the sampling of the frequency channels in the second subset; and
transmitting synchronization information sequentially to each of a plurality of frequency channels in a third subset of the frequency channels, the third subset including one of the frequency channels in the second subset and none of the frequency channels in the first subset.
15. A method of synchronizing wireless devices in a wireless network, the method comprising the steps of:
informing each of the wireless devices of a recurring sequence of frequency channels at which the wireless devices are to communicate non-synchronization information;
transmitting synchronization information sequentially to each of a plurality of frequency channels in a first subset of the recurring sequence of frequency channels;
using one of the wireless devices to sample each of the frequency channels in a second subset of the recurring sequence of frequency channels, the second subset including one frequency channel included in the first subset and at least one frequency channel omitted from the first subset;
transmitting non-synchronization information after the sampling of the frequency channels in the second subset; and
transmitting synchronization information sequentially to each of a plurality of frequency channels in a third subset of the recurring sequence of frequency channels, the third subset including one of the frequency channels in the second subset and none of the frequency channels in the first subset.
7. A method of synchronizing wireless devices in a wireless network, the method comprising the steps of:
pre-programming each of the wireless devices with a list of frequency channels and a time duration for which non-synchronous information is to be transmitted to each of the frequency channels in the list;
transmitting synchronization information sequentially to each of a plurality of frequency channels in a first subset of the frequency channels in the list;
using one of the wireless devices to sample each of the frequency channels in a second subset of the frequency channels in the list, the second subset including one frequency channel included in the first subset and at least one frequency channel omitted from the first subset;
transmitting non-synchronization information after the sampling of the frequency channels in the second subset; and
transmitting synchronization information sequentially to each of a plurality of frequency channels in a third subset of the frequency channels in the list, the third subset including one of the frequency channels in the second subset and none of the frequency channels in the first subset.
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This application is a continuation of U.S. patent application Ser. No. 12/188,417 entitled “METHOD FOR FAST SYNCHRONIZATION AND FREQUENCY HOP SEQUENCE DETECTION IN WIRELESS SENSOR NETWORKS”, filed Aug. 8, 2008, the complete subject matter of which is hereby incorporated herein by reference, in its entirety.
Portions of this document are subject to copyright protection. The copyright owner does not object to facsimile reproduction of the patent document as it is made available by the U.S. Patent and Trademark Office. However, the copyright owner reserves all copyrights in the software described herein and shown in the drawings. The following notice applies to the software described and illustrated herein: Copyright© 2011, Robert Bosch GmbH, All Rights Reserved.
1. Field of the Invention
The present invention relates to frequency hopping wireless systems, and, more particularly, to synchronization of frequency hopping wireless systems.
2. Description of the Related Art
According to FCC regulations, any short-range wireless system that operates in frequency bands 902-928 MHz or 2400-2483.4 MHz should employ either frequency hopping methods or use digitally modulated techniques. Similar requirements are present in European standards (EN300-220) regarding the use of frequency band 863-870 MHz.
In the frequency hopping methods, the system divides the available frequency bandwidth into many frequency channels, and it continuously switches the channel, using a pseudorandom frequency hop sequence known to both the transmitter and the receiver. The FCC regulation for the 902 MHz band requires the system to use a minimum of fifty frequency channels. The nodes in the network follow a pseudorandom frequency hop pattern as the system switches between different frequency channels. The period during which the system stays in one frequency channel before hopping to the next channel is referred to as “dwell time”. The FCC and EN300-220 regulations specify a maximum dwell time of 400 milliseconds. The dwell time can be fixed or it can change for different transmissions. However, each transmitter should, on average, use each channel in the frequency hop sequence equally. In addition, the FCC regulation specifies a maximum channel usage by the system: The average time of occupancy on any frequency channel should not be greater than 400 milliseconds within any window having a length of twenty seconds.
Although both the sender and the receiver (and in general all the nodes in the system) are assumed to know the frequency hop sequence, in order to have a successful communication they should also be in synchronization in the sense that they should both know which position in the frequency hop sequence is being used at any moment in time. The system should provide a mechanism for the newly added nodes, or existing nodes that have lost the time-synchronization, to get in synchronization with the rest of the network and find the current frequency hop index in the frequency hop sequence.
Synchronization latency is an important design factor. A newly added node should be able to get in synchronization with minimal delay to start the communication with the remainder of the network. The acceptable latencies for the synchronization process in many applications are on the order of one to two seconds.
On the other hand, it is desired to use as few transmissions as possible for the synchronization process. This is important because the regulation imposes a limit on the period of time that each channel can be used by the system (i.e., 400 milliseconds in any window of twenty seconds). The less transmission time that is used for the synchronization process, the more time remains for other network operations.
What is neither disclosed nor suggested by the prior art is a synchronization method that enables fast synchronization while, on average, the transmission time on each channel is reduced.
The present invention addresses the synchronization problem in frequency hopping systems. Particularly, the invention provides a solution for synchronizing, in a time-efficient manner, a newly added node or an existing node that has lost time-synchronization with the rest of the network.
The invention comprises, in one form thereof, a method of synchronizing wireless devices, including establishing a recurring sequence of frequency channels at which the wireless devices are to communicate. The frequency channels are divided into a plurality of groups. Synchronization information is transmitted at a respective first frequency channel in each of the groups of frequency channels during a first sampling time period. One of the groups of frequency channels is selected. A wireless device is used to sample each of the frequency channels in the selected group during the first sampling time period. Non-synchronization information is transmitted after the first sampling time period. Synchronization information is transmitted at a next respective frequency channel in each of the groups of frequency channels during a next sampling time period. The next sampling time period occurs after the transmitting of the non-synchronization information.
The invention comprises, in another form thereof, a method of synchronizing wireless devices, including establishing a recurring sequence of frequency channels at which the wireless devices are to communicate. The frequency channels are divided into a plurality of groups. Synchronization information is transmitted at a respective first frequency channel in each of the groups of frequency channels during a first sampling time period. A wireless device is used to select one of the groups of frequency channels. The wireless device is used to sample each of the frequency channels in the selected group for the synchronization information. The sampling occurs during the first sampling time period. Non-synchronization information is transmitted after the first sampling time period. Synchronization information is transmitted at a next respective frequency channel in each of the groups of frequency channels during a next sampling time period. The next sampling time period occurs after the transmitting of the non-synchronization information. Non-synchronization information is transmitted after the next sampling period. The steps of transmitting synchronization information at a next respective frequency channel and transmitting the non-synchronization information are repeated until synchronization information has been transmitted in each frequency channel in each of the groups of frequency channels.
The invention comprises, in yet another form thereof, a method of synchronizing wireless devices, including informing each of the wireless devices of a frequency channel hop sequence at which non-synchronization information is to be transmitted. Synchronization information is sequentially transmitted to each of a plurality of frequency channels in a first subset of the frequency channels included in the hop sequence. A wireless device is used to sample each of the frequency channels in a second subset of the frequency channels. The second subset includes one frequency channel included in the first subset and at least one frequency channel omitted from the first subset. Non-synchronization information is transmitted after the sampling of the frequency channels in the second subset. Synchronization information is sequentially transmitted to each of a plurality of frequency channels in a third subset of the frequency channels. The third subset includes one of the frequency channels in the second subset and none of the frequency channels in the first subset.
An advantage of the present invention is that both the synchronization latency and the transmission time on each frequency channel are reduced.
Another advantage is that the present invention complies with all known government regulations for frequency channel usage.
The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed.
The present invention may be described herein in terms of algorithms and operations on data bits within a computer. It has proven convenient, primarily for reasons of common usage among those skilled in the art, to describe the invention in terms of algorithms and operations on data bits. It is to be understood, however, that these and similar terms are to be associated with appropriate physical elements, and are merely convenient labels applied to these physical elements. Unless otherwise stated herein, or apparent from the description, terms such as “calculating”, “determining”, “processing” or “computing”, or similar terms, refer the actions of a computing device that may perform these actions automatically, i.e., without human intervention, after being programmed to do so.
Referring now to the drawings and particularly to
Base station 22 and control panel 30 may be powered by household alternating current, and sensors 241, 242, . . . , 24n, siren 26 and key fob 28 may be battery powered. For sensors 241, 242, . . . , 24n, siren 26 and key fob 28, base station 22 is the gateway to control panel 30, which the user can use to interact with the system. In one embodiment, network 20 is in the form of a wireless Local Security Network (wLSN) system which is a wireless intrusion and alarm system.
The network uses a frequency hop pattern. After each transmission, the network hops to the next channel in the hop sequence.
In each “beacon slot” many announcement messages or “announcement packets” may be transmitted. The announcement packets may contain information about the hop pattern and the current frequency hop index. For example, the announcement packets may contain the seed value of the random number generator which is used for creating the pseudorandom frequency hop pattern.
The beacon transmissions need to comply with the government regulations, and therefore it may be advantageous for the beacon transmissions to be sent to all frequency channels used by the system. Thus, the beacons are generally not transmitted to only one fixed frequency channel or only one sub group of channels. It may be advantageous for the announcement packets (beacons) to themselves follow a frequency hop sequence and use each frequency channel equally. The beacon hop sequence can be the same as or different from the main frequency hop pattern used in the network. However, it is possible, according to the invention, for the beacon hop sequence to be different from the main frequency hop pattern used in the network in order to improve the synchronization latency.
The upper portion of
The synchronization latency may be determined by how often the system sends the beacons, i.e., the time period between time-adjacent transmissions of the beacon slots. This time period is labeled in
A problem with the embodiment depicted in
With the assumption of fifty-nine different frequency channels being used, the transmission of a beacon slot may require fifty-nine data packet transmissions at twelve milliseconds per transmission, i.e., 708 milliseconds. The beacon slot may be transmitted once per second in order to achieve a worst-case delay of one second for synchronization. Thus, on average, in each one second time period, 708 milliseconds (70.8% of the time) is occupied by beacon slot transmissions for synchronization, and the remaining 292 milliseconds (29.2% of the time) is available for other network activities, as is illustrated in
Another factor that may be considered in view of the government regulations is the amount of time that may be used for synchronization within each individual frequency channel. The regulations specify that within any window of twenty seconds duration, each channel may be used (for any purpose, including synchronization and/or other network operations) for a maximum of 400 milliseconds. The transmission of announcement packets occupies each channel for twelve milliseconds in each one-second period, and so, in a twenty second period, each channel is used for 240 milliseconds (12 milliseconds/second×20 seconds) by the beacon slots for the transmission of announcement packets. This leaves only 160 milliseconds (400 milliseconds−240 milliseconds) for other network activities.
Another embodiment of a synchronization method of the present invention, as described below, has the advantages of achieving synchronization in less time and using the frequency channels more efficiently. In this embodiment, the frequency channels are grouped, and longer but fewer announcement packets are transmitted. This may have the effect of reducing the transmission time of beacons and reducing the frequency channel usage.
In this embodiment of a method for transmitting the beacons, the frequency channels are divided into groups. In the specific embodiment to be next described, the frequency channels are divided into groups of two for ease of illustration herein. After the simple case of groupings of two frequency channels is described, the generalized case of arbitrary groupings of arbitrary size will be described herein.
As mentioned above, all of the frequency channels (in this case, fifty-nine frequency channels) utilized by the network may be divided into groups of size two as follows:
Because the receiving node is sampling both channel 1 and channel 2, the base station does not need to send announcement packets in both channels in order for the node to receive the announcement packet. In each beacon slot, the base station transmits to one channel or the other in each group of two frequency channels. In a particular embodiment, in each beacon slot, the base station transmits in either the even-numbered channels or the odd-numbered channels.
With the embodiment illustrated in
The amount of time needed by the radio to switch to a new channel and sample the signal level may be about three to five milliseconds, as illustrated in
In the embodiment illustrated in
As shown in
In general, according to an embodiment of the present invention, the frequency channels may be divided into groups of size n. The receiving node may select one of the groups randomly and sample all of the n channels back-to-back. As the number of frequency channels in a group increases, the length of time needed by a node to sample all of the frequency channels in the group increases accordingly. Thus, the Added Preamble may be correspondingly lengthened to match the increased length of time between samples of a same frequency channel by a single node.
In order to select advantageous values for n, two parameters may be considered:
As may be seen in the plots of
In each beacon slot, the base station sends three 112 millisecond long announcement packets (100 millisecond preamble and 12 millisecond packet). Thus, the beacon slots are 336 milliseconds long (3×112 milliseconds), leaving 664 milliseconds available for other network operations, as illustrated in
In a next step 1204, the frequency channels are divided into a plurality of groups. For example, in the embodiment of
In step 1206, synchronization information is transmitted at a respective first frequency channel in each of the groups of frequency channels during a first sampling time period. Synchronization information may include time periods and specific frequency channels associated with transmission of non-synchronization information. That is, synchronization information may include a schedule of frequency channels and times at which non-synchronization information is transmitted. It is also possible, within the scope of the invention, for synchronization information to include a schedule of frequency channels and times at which synchronization information is transmitted. In the embodiment illustrated in
In a next step 1208, a wireless device is used to select one of the groups of frequency channels. For example, in the embodiment of
Next, in step 1210, one of the wireless devices is used to sample each of the frequency channels in the selected group for the synchronization information, the sampling occurring during the first sampling time period. In the embodiment of
In step 1212, non-synchronization information, i.e., information other than synchronization information, is transmitted after the first sampling time period. In the embodiment of
Next, in step 1214, synchronization information is transmitted at a next respective frequency channel in each of the groups of frequency channels during a next sampling time period, the next sampling time period occurring after the transmitting of the non-synchronization information. In the embodiment illustrated in
In a next step 1216, non-synchronization information is transmitted after the next sampling period. In the embodiment of
In step 1218, it is determined whether synchronization information has been transmitted in each of the frequency channels in each of the groups of frequency channels. In the embodiment of
The description of step 1218 above includes references of rounds of synchronization information transmission through all of the frequency channels. It is to be understood that the present invention does not place any restrictions either on the number of wireless devices that may sample the frequency channels for synchronization information or on the times at which the wireless devices may do the sampling. For example, any number of wireless devices may simultaneously sample the same or different frequency channels. In general, the transmission of synchronization information and non-synchronization information by the hub may be unaffected by the sampling the transmissions by any number of wireless devices at any times.
In a next step 1304, synchronization information is sequentially transmitted to each of a plurality of frequency channels in a first subset of the frequency channels included in the hop sequence. For example, in the embodiment of
In step 1306, one of the wireless devices may be used to sample each of the frequency channels in a second subset of the frequency channels, the second subset including one frequency channel included in the first subset and at least one frequency channel omitted from the first subset. In the embodiment of
Next, in step 1308, non-synchronization information is transmitted after the sampling of the frequency channels in the second subset. In the embodiment of
In a final step 1310, synchronization information is sequentially transmitted to each of a plurality of frequency channels in a third subset of the frequency channels. The third subset includes one of the frequency channels in the second subset and none of the frequency channels in the first subset. In the embodiment of
The present invention has been described herein as being applied to synchronizing wireless devices in a particular frequency band. However, it is to be understood that the invention may also be applicable to synchronizing wireless devices that operate in other frequency bands.
While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.
Keshavarzian, Abtin, Manjeshwar, Arati
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